Separation of gas oil in coking residua



Dec. 23, 1958 J. F. MOSER, JR 2,865,840

SEPARATION OF GAS OIL IN COKING RESIDUA Filed NOV. 25, 1953 FRAOTIONATOR SCRUBBER STEAM WATER John F. Moser Jr. Inventor By )1? m Attorney 2,865,840 Patented Dec. 23

United States PatentfQfiF ce 2,865,840 SEPARATION OF GAS OIL IN COKING RESIDUA Application November 25, 1953, Serial No. 394,424

1 Claim. (Cl. 208-97) The present invention relates to an improvement in the separation of gas oil in coking resid'ua, particularly in -coking processes where residua are contacted with heat carrying particles to convert them to lower boiling prod- .ucts and coke.

It has been previously suggested that heavy residual .oils can be converted to lower boiling products, with accompanying production of a certain amount of coke, by contacting such oils with a mobile mass of preheated solid particles. A preferred process is one wherein small solid particles of good heat carrying properties are contacted with the residuum in such a manner as to evaporate and crack the major constituents thereof, resulting in the production of gas oil and some lower boiling materials with conversion of the remainder to coke. While various solids can be employed, coke particles which are produced in the process are very satisfactory and are commonly preferred.

The preheated solid particles, for example coke particles which may range in size between about 40 and 400 microns average particle diameter, are preheated to a temperature between 1000 and 1500 1100 and 1300 F. and a stream of such preheated particles is introduced into a coking reactor. In the reactor the preheated particles are mobilized by passing a stream of'gas or gasiform fluid upward through the mass to fluidize the particles. The residual oil feed, usually preheated moderately, is introduced into the bed of fluidized particles with the result that the feed is vaporized and cracked with the vaporized products passing overhead into suitable recovery apparatus.

F usually between In coking processes where gas oil is desired for subsequent feeding to a catalytic cracking operation, considerable difliculty has been encountered in separating the highly objectionable heavy fractions which contain metalr lic compounds such as salts of vanadium, nickel and the like, from the more desirable gas oil components.

The latter usually boil at a slightly lower temperature than the objectionable metallic constituents, but it has been-extremely difficult to obtain sharp separation. Consequently it is usually necessary to separate a bottoms fraction from thegas oil products which fraction includes a considerable quantity of gas oil within the desirable boiling range for feed to a catalytic cracking operation. Thus, in order to avoid contaminating the cracking catalyst, it has been necessary in the past to discard a considerable proportion of the desirable gas oil product from the coker. Alternatively, it has been suggested that the wide cut bottoms fraction can be returned to the coker. By recycling the bottoms the gas oil product can be maximized without entraining objectionable quantities, of the catalyst contaminants in the heavy ends. Unfortunately, the recycling of gas oil through a coke bed of fluidized particles tends to convert a substantial part of such gas oil to dry gas products of low economic value. In other words, the recycle gas oil passing through the coker bed is over-cracked to an objectionable degree.

According to the present invention it has been discovered that the wide cut bottoms fraction containing substantial proportions of gas oil of a desirable boiling range may be treated under mild coking conditions separately from the original feed in such a manner as to recover practically all of the gas oil without carrying the contaminants in the heavy ends over into thegas oil product stream. It is therefore an object of the present invention to treat a wide cut bottoms fraction'from the coker efiluent in such a manner as to recover the gas oil constituents thereof without substantial entrainment of catalyst contaminants. This is accomplished by passing the bottoms fractions into a stream'of partially quenched solid particles or into a stream of such particles which are substantially lower in temperature than the major mass of particles in the primary coking bed. The latter should be from 25 to 250 F. lower in temperature than the primary bed.

In effect, the bottoms fraction from the primary coking operation is treated thermally by contacting it briefly with heat carrying solids at moderate temperature to partially vaporize it, leaving the unvaporized portions of the condensed bottoms on the solid particles while the more volatile gas oil fractions are taken into the product recovery system along with the primary coker products. Thus, in accordance with the present invention, a heavy condensate is separated from the vapor effiuent which comprises the primary coker product from a fluid bed coking operation. This condensate is subjected to a relatively low temperature vaporizing and coking treatment separate from the treatment of the fresh feed.

Another feature of the present invention involves the withdrawal of a stream of heat carrying solids from the primary coking zone, the partial quenching of such associated therewith is revaporized by the mild treatment without converting it to gas as would occur if the bottoms were returned to the primary coker. Thereafter the solids particles carrying the deposited heavy ends and catalyst contaminants are returned to the coking zone while the gas oil products are taken to the recovery system without re-entering the coker zone.

An important object of the present invention, therefore, is to accomplish a vaporization of the gas oil from the primary coker bottoms, without appreciable coking there of.

The invention will be better understood by referring in detail to the accompanying drawing wherein a preferred embodiment of the invention is shown diagrammatically in elevation.

Referring to the attached drawing, a stream of preheated solid particles such as coke particles. of a size that can readily be fluidized is led into a reactor vessel ,11 to form a fluidized bed 13. A stream of feed stock such as a heavy petroleum residuum is introduced through a line 15 and dispersed into the bed 13 by appropriate nozzles which are not shown since they form no part of the present invention. The solid particles, at a suitable temperature to effect the coking reaction, cause the feed to be vaporized and cracked. The bed temperature is between 900 and 1100 F;, preferably 950 to 1025 F. The vapor products pass overhead to acyclone separator 17 from which the entrained solid particles are returned to the bed through a solids return line 19. The vapor products, now'substantially free from solid particles, pass overhead to suitable recovery equipment which preferably includes a scrubber 2.1 of conventional design equipped with suitable baffle means 23 which may be of the disc and donut type.

Superimposed on the scrubber 21 is a fractionator 25,

the outer structure for-the two preferably comprising a single unitary vessel. In the fractionator 25 the vaporized products are separated into the desired fractions, such as a gas taken off through line 27, a motor fuel or naphtha through line .29, a gas oil through line 31 and a bottoms fraction through line 33. Scrubber bottoms are withdrawn through line 34.

Near the bottom of the coker bed 13 a withdrawal line 35 carries a stream of fluidized particles downwardly through a U-bend or other return bend 37 and upwardly into a riser 39 which constitutes a mild vaporizing zone.

-A lift gas consisting of a mixture of steam and water is introduced through line 41 into the bottom of the riser :39. This partially quenches the hot solids coming from the coker and reduced the temperature moderately, for example from an original temperature of 900-1000 F.

down to about 750900 F. The steam, including the water which is quickly evaporated, provides sufficient lifting gas to carry the solids upwardly through the riser The bottoms fraction from the tractionator is fed through line 33 into this rising stream of solids where it is distributed onto the solids particles and carried upwardly. The solids still contain sufficient heat for vaporizing the gas oil fraction of the bottoms but not to vaporize the heavy fractions which carry the metallic contaminants. The latter are deposited on the surface of the particles as they flow upwardly. The contact time in this mild vaporizing zone is preferably shorter than in the main coker bed.

The products from the riser or mild vaporizing zone 39 are carried into a separator or cyclone 45 from which the vapor'products pass overhead through line 47 into the scrubber 21. The solids, carrying the metallic contaminants and other heavy end constituents, pass downwardly through line 49 and are returned to the bed 13 fairly near the bottom thereof. With this arrangement they are thoroughly mixed into the bed so that further cracking of the heavy bottoms constituents may be accomplished to an extent sufiicient to prevent agglomeration of the particles in the bed.

The spent coke particles in the coker bed 13 pass downwardly through a stripping zone 51' located at the bottom of reactor 11. A stripping gas such as steam or hydrocarbon gas may be introduced through a line 53 topass upwardly in counterfiow to the spent solids. The latter pass down through outlet line 55 to a return bend 57 and upwardly through line 59 into a burner vessel 61. A stream of air or other oxidizing gas may be introduced through a line 63 under control of valve 65 to carry the spent solids upwardly into the burner and to cause them to burn and be reheated. Preferably a part of the air is introduced directly into the bottom of the burner through a branch line 67 under control of valve 69.

,In the burner the combustion gases are taken overhead through a separator or cyclone 71, the entrained solids being returned through a dipleg 73, while the flue gasespassoverhead through line 75. The reheated coke overflows a partition-or weir 77 and returns through a standpipe line 79 to a return bed 81 and a riser 83 which extends as apartition 84 substantially to the top of the coker bed 13 in reactor 11. With this arrangement the hot solids are returned to the top of the coker bed and the'spent solids withdrawn at the bottom, with the sidestream taken oif and returned throughlines 35, 39 and 49 being withdrawn from and returned to the lower part ofthe coking bed proper.

This arrangement has the following advantages:

(1) The recycled gas oil is not subjected to temperatures high enough or at times long enough to crack it substantially'to gas, thereby increasing the yield of gas oil and minimizing losses to gas.

(2) Since the bottoms recycle is introduced into a'high velocity riser, no complicated atomizing nozzle is required.

(3) Since the separation temperature is low, the heavy bottoms are retained on the solids returned to the coker.

(4) The flashed gas oil provides a good deal of the gas velocity required in the riser which constitutes the mild vaporizer, thereby reducing the diluent steam needed.

(5) The heavy bottoms are returned near the bottom of the coker so that their conversion to lighter products is maximized, time being thus provided for full mixing into the bed.

(6) The solids circulating side stream provides additional turbulence to break up and/or reduce bed agglomeration.

(7) The transfer line which serves as a mild vaporizer for gas oil included with the heavy ends is an economical apparatus for effecting short time contacting with solids at low temperature without bed bogging and related troubles.

What is claimed is:

A process of coking residual hydrocarbon oils containing objectionable 'high boiling metallic compound impurities which comprises the steps of contacting the residual oil in a coking zone with a dense, turbulent, fluidized bed of preheated particulate solids at a temperature in the range of 900 F. to 1l00 F., to crack the hydrocarbon oil and to vaporize gas oil and heavy ends boiling above the gas oil range which heavy ends include said objectionable metallic compounds, condensing and separating from the vapors so produced in a scrubbing-fractionating zone a scrubber liquid bottoms fraction and a separate fractionator bottoms fraction, said fractionator bottoms fraction being of sufiiciently low boiling range to include substantially all said heavy ends and objectionable metallic compounds and also to include some gas oil substantially free of said objectionable metallic compounds, removing product vapors overhead,

withdrawing hot particulate solids from said coking zone, passing the withdrawn solids into the lower portion of a vertically arranged tubular solids contacting zone, partially quenching the withdrawn solids with water in the lower portion of said tubular contacting zone to a temperature in the range of 25 to 250 F. lower than in said coking zone to form steam to act as a lifting vapor for the solids, introducing the separated fractionator bottoms fraction and the scrubber liquid bottoms fraction into the lower portion of said tubular contacting zone above the point of introduction of said water and hot particulate solids for contact with said solids so as to vaporize predominantly only the included gas oil substantially free of objectionable .metal compounds without appreciable coking there of and to deposit the high boiling heavy hydrocarbon ends including most of the objectionable metallic compounds on said introduced particulate solids while passing the entire mixture of solids and vapors upwardly at a high velocity through said tubular contacting zone, then pa'ssingsaid mixture of solids and vapors from the top of said tubular contacting zone into a solids-separating z'on'e to separate solids from vapors, passing said separated vapors to the scrubbing portion of said scrubbing-tractionating zone below the bottom of the fractionating portion of said scrubbingfractionating zone and passing said separated solids with the deposited heavy ends containing said objectionable metal compounds to a lower portion of said coking zone for further cracking of said heavy ends.

References Cited in the file of this patent UNITED STATES PATENTS 

